1. Field of the Invention
This invention relates to the field of analog to digital converters.
2. Background Art
It is often desired to convert an analog signal to a digital signal. For converting electrical signals, an analog to digital (A/D) converter is used. There are several types of A/D converters used in the prior art.
One simple method of A/D conversion is known as a tracking converter in which an analog input signal is coupled to a comparator along with with output of a digital to analog (D/A) converter. The output of the comparator is coupled to an up/down counter and the output of the counter is coupled to the input of the D/A converter. When the analog input is higher than the output of the D/A converter, the counter counts up. When the analog input is lower than the D/A converter, the counter counts down. The output of the converter is stable when a change of one count up or down causes the comparator output to reverse sign.
Another type of prior art A/D converter is an integrating converter such as a dual slope integrator. An analog input signal is coupled to a three position switch, where one position is grounded and the remaining contact is connected to a stable reference voltage. The output of the switch is connected to an analog integrator with a time constant TL. The output of the integrator drives a comparator whose output is coupled to a control circuit. The control circuit is coupled to the input switch and switches the input from the analog input to ground and to the reference voltage VR. The control circuit is also coupled in a loop with a digital counter.
This type of A/D converter works in three steps. In the first step, the input switch is coupled to ground and the counter is preset to a number N.sub.O. The integrator is also set to zero during this stage. In the second stage, the switch is coupled to the input signal, and the counter and integrator are released. The counter begins to count down towards zero and the integrator voltage grows at A, dependent on the input line voltage.
After the counter has reached zero, the control circuit switches the switch to the reference voltage and begins counting up from zero. The integrator voltage is now dependent on the reference voltage VR. When the integrator voltage reaches zero, the comparator output turns off the counter. The count accumulated in the counter at the end of this time interval is proportional to the analog input signal.
Another type of A/D converter is known as a successive approximation converter. One type of this converter employs a scheme known as charge redistribution and utilizes an array of capacitors or resistors or both, to provide ranges of values to define the level of an analog input signal. For example, a variety of capacitors are provided whose capacitance values are in a binary relationship. That is, if the capacitance of the lowest capacitor is C farads, capacitors having capacitance values of 2C, 4C, 8C, 16C, 32C, etc., are also provided. The greater the number of capacitance values, the more precise the conversion of analog to digital signals can be achieved.
To provide an A/D converter having 8-bit precision, at least 3 levels of capacitance, C, 2C, and 4C, are provided. In this manner, through an appropriate switching scheme, capacitance levels of C, 2C, 3C (C+2C), 4C, 5C (4C+C), 6C (4C+2C) and 7C (4C+2C+C) can be defined.
The linearity of such a capacitor value scheme depends upon the accuracy of the ratios of the capacitors to each other. If the capacitor ratios are not the desired integer ratios, non-linearity in the output of the A/D converter results.
It is desired for the output of an A/D converter to be substantially linear. In the prior art, particularly in switched capacitor A/D converters, converter linearity depends on the matching of resistors and capacitors. Such matching of capacitors and resistors adds to the expense of manufacturing such A/D converters.
Therefore, it is an object of the present invention to provide an A/D converter which provides a linear output without the need for precise matching of individual components.
It is another object of the present invention to provide an A/D converter in which the effect of mismatched components can be averaged out and substantially eliminated.